Aromatic hydrocarbon compounds such as benzene, toluene, and xylenes (“BTX”) are frequently used for producing transportation fuels and petrochemicals such as styrene, phenol, nylon and polyurethanes and many others. Processes have been developed for producing light aromatic hydrocarbon from relatively inexpensive feeds, e.g., from paraffinic C4− feeds. The processes typically are carried out using a catalyst comprising molecular sieve, such as ZSM-5 and at least one dehydrogenation metal. These conventional processes typically operate at high temperature and low pressure, which can lead to excessive catalyst coking. Catalyst coking generally worsens under conditions which increase feed conversion, leading to additional operating difficulties.
Processes have been developed for converting less-refractory paraffinic hydrocarbon to aromatic hydrocarbon with decreased selectivity for catalyst coke. For example, U.S. Pat. No. 4,855,522 discloses converting C2, C3, and C4 hydrocarbon with increased selectivity for aromatic hydrocarbon and decreased selectivity for catalyst coke. The process utilizes a dehydrocyclization catalyst comprising (a) an aluminosilicate having a silica:alumina molar ratio of at least 5 and (b) a compound of (i) Ga and (ii) at least one rare earth metal. The reference discloses carrying out the aromatization conversion at a space velocity (LHSV) in the range of from 0.5 to 8 hr−1, a temperature ≧450° C. (e.g., 475° C. to 650° C.), a pressure of from 1 bar to 20 bar, and a feed contact time of 1 to 50 seconds.
More recently, catalysts have been developed to further reduce the amount of catalyst coking during the dehydrocyclization of C4− paraffinic hydrocarbon. For example, U.S. Pat. No. 7,186,871 discloses increasing the catalyst's dehydrogenation metal loading to lessen the amount of catalyst coking. It has been found, however, that doing so increases the catalyst's hydrogenolysis activity, resulting in an increase in the amount of methane and other light saturated hydrocarbon in the reaction product and a decrease in the amount of the desired aromatic hydrocarbon. This effect can be mitigated by further increasing catalyst complexity, e.g., by adding an attenuating metal to the catalyst as disclosed in U.S. Pat. No. 8,692,043.
Hydrogenolysis side-reactions can also be mitigated by carrying out the aromatization in two stages. For example, U.S. Pat. No. 8,835,706 discloses aromatization of an ethane-propane feed. The feed, which is obtained from natural gas by cryogenically separating methane, is reacted in a first stage operated under conditions which maximize the conversion of propane to aromatics. In one embodiment, ethane is separated from the first stage's reaction product. The separated ethane is reacted in the second stage to produce aromatics. In another embodiment, aromatics are separated from the first stage's reaction product. In both embodiments, ethane is reacted in the second stage to produce aromatics. The second stage is operated under conditions which maximize the conversion of ethane to aromatic hydrocarbon. The process can be operated continuously by cycling between first and second reactors located in each stage. The first reactor carries out aromatization (reaction mode) while the second reactor undergoes decoking (regeneration mode), and vice versa. The patent discloses that increased catalyst coking can be overcome by utilizing fluidized catalyst beds in the reaction stages. Decreasing the amount of time (the “cycle time”) that a fixed bed reactor is operated in reaction mode before switching to regeneration mode can also be used to lessen the amount of coke accumulation.
Improved processes are needed for dehydrocyclization of light paraffinic hydrocarbon that exhibit one or more of a greater feed conversion, a greater yield of aromatic hydrocarbon, and a lesser yield of undesired byproducts such as catalyst coke and C4− hydrocarbon. Processes are particularly desired which can be carried out with catalysts of lesser complexity, in fixed catalyst beds with increased cycle time, and/or with a decreased need for light gas separation.